1. Field of the Invention
This invention generally relates to the field of heat dissipation. More specifically, the invention relates to a thermosyphon that enhances cooling of electronic systems.
2. Description of the Problem
Cooling of electronic circuit components in thin space enclosures is often performed by metal plates that spread heat, referred to as heat spreaders. Examples of devices where heat spreaders are used include portable computers, high-speed memory modules, inkjet printers, and some handheld devices. A heat spreader's internal thermal resistance, which is a measure of the heat removal performance of a device, increases as the spreader thickness decreases. Size reductions of electronic systems make thinner spreaders necessary. Increasing the thermal conductivity of the spreader can offset the resulting increased internal resistance. One way to achieve very high effective thermal conductivity is to use a fluid-filled cooling device that takes advantage of the heat of vaporization of the fluid by transporting heat from an evaporator to a condenser through the liquid-vapor phase change. Two known types of devices in particular employ this phase-change mechanism for heat transfer from electronic circuit components: thermosyphons and heat pipes.
Thermosyphons are fluid-filled closed loop devices, incorporating an interconnected evaporator and condenser. The working fluid undergoes a liquid to vapor phase change in the evaporator, thereby absorbing the latent heat of vaporization. The vapor then travels to the condenser, where the heat is lost to the environment and the cooled working fluid condenses to liquid The evaporator is typically oriented vertically with respect to the electronic circuit component to be cooled. The performance of an entirely passive system, where there are no moving parts, requires the condenser to be located vertically above the evaporator. The use of known thermosyphons is therefore limited to enclosures that can accommodate and remain fixed in this required orientation.
Heat pipes are hollow sealed devices, containing a wick structure saturated with a working fluid. Despite their name, which came from the geometry of the early forms of the devices, heat pipes may be any shape. In their early shape that resembles a pipe, heat is transferred from electronic circuit components, typically the processor chip, to the working fluid in the evaporator portion at one end of the heat pipe. The working fluid undergoes a liquid to vapor phase change in the evaporator portion, thereby absorbing the latent heat of vaporization. This heat is carried by the working fluid to the other end of the pipe, which is the condenser portion, and is rejected to the environment. The cooled working fluid vapor condenses, and urged by the surface tension forces that are generated by the wick structure, returns to the evaporator portion.
Current known heat pipe structures include flat-plate heat pipes, where heat may be added at any location. The working fluid evaporates, moves to lower pressure and cooler regions of the cavity, and is cooled on the walls of the heat pipe where it condenses. In recently developed micro heat pipes, microfabricated grooves replace the wick structure and provide the capillary action for the return of the condensed vapor to the evaporator portion of the device.
While heat pipes do not have the geometric orientation constraints of thermosyphons and are an improvement over known heat spreaders, their performance is limited. A significant drawback of the heat pipe comes from its very mechanism, that is, capillary driving of the condensate that makes the heat transfer performance orientation-independent. The capillary action in the heat pipe is based on the thinness of liquid film in the wicking structure, and the difference in liquid/vapor menisci in the condenser and the evaporator. If the liquid film is thick, gravity comes to influence the liquid flow, and the heat pipe performance becomes orientation-dependent. Liquid evaporates as the condensate flows toward the middle of the evaporator section in a flat heat pipe, or toward the end of the evaporator section in a cylindrical heat pipe. The circulation rate of the working fluid in the heat pipe is constrained where the liquid film thickness reduces to zero due to evaporation. A part of the evaporator surface dries, and the surface temperature then increases beyond a level acceptable for the application. This so-called “capillary limit” restricts the application of heat pipes to cases of moderate chip heat dissipation and relatively small heat spreader areas. Larger heat spreader areas inherently have longer wicking structure length, and hence there is more potential for poor performance as a result of the capillary limit. Better thermal performance is desirable to meet the cooling requirements of increasingly faster electronic circuit components.
There is a need for a device that has superior cooling performance while eliminating the orientation constraints of known thermosyphons. Ideally, the device will be generally orientation-independent, and will be compact in size as necessary to meet thin space enclosure requirements.